i would say that it does flex. However your idea of rigid could be my idea of flexible.
It is fair to say that its better to classify it as simple FRP. Polyurethane is what i would call flexible. Further i find flexiability almost non important as it is a rear bumper and something that will not take constant scraping as micra has very short overhang. The front bar is more important place to consider whether the bar flexs or not.